Identifying Dark Matter Haloes by the Caustic Boundary

TL;DR

This paper introduces a geometric method to identify dark matter haloes by detecting caustic boundaries through triangulation of particle data, avoiding reliance on density peaks and assumptions about halo shape.

Contribution

A novel geometric algorithm for caustic detection in N-body simulations that accurately identifies dark matter haloes without specific geometric assumptions.

Findings

Halo boundaries are asymmetrical and not spherical or ellipsoidal.

The identified haloes are gravitationally bound.

The method effectively detects caustics using triangulation of Lagrangian sub-manifolds.

Abstract

Dark matter density is formally infinite at the location of caustic surfaces, where dark matter sheet folds in phase space. The caustics separate multi-stream regions with different number of streams. Volume elements change the parity by turning inside out when passing through the caustic stage. Being measure-zero structures, identification of caustics via matter density fields is usually restricted to fine-grained simulations. Instead a generic purely geometric algorithm can be employed to identify caustics directly by using triangulation of Lagrangian sub-manifold x(q, t) where x and q are Eulerian and Lagrangian coordinates obtained in N-body simulations. The caustic surfaces are approximated by a set of triangles with vertices being the particles in the simulation. It is demonstrated that finding a dark matter halo is quite feasible by building its owtermost convex caustic. Neither more specific assumptions about the geometry of the boundary nor ad hoc parameters are needed. The halo boundary in our idealized but undoubtably generic simulation is neither spherical nor ellipsoidal but rather remarkably asymmetrical. The analysis of the kinetic and potential energies of individual particles and the halo as a whole along with an examination of the two-dimensional phase space has shown that the halo is gravitationally bound.